1. Technical Field
The present invention relates to a technology for preventing image sticking in a case in which a so-called “region-scanning driving method” is employed in a liquid crystal device.
2. Related Art
In recent years, projectors which use liquid crystal devices to form small images and which use optical systems to project images formed by enlarging the small images have been in widespread use. In a liquid crystal device for forming a small image, so-called “disclination (misorientation)” can be a problem since a distance between pixels is very small. The disclination can be avoided by employing in-plane inversion (also called “frame inversion”) in which pairs of two adjacent pixels are set to have the same polarity. However, the in-plane inversion has a problem in that, for example, top and bottom ends on a displayed screen have a display difference.
To eliminate the display difference, so-called “region-scanning driving” (see, for example, JP-A-2004-177930) has been proposed. In the region-scanning driving, by dividing the period of a frame into, for example, first and second fields, and performing positive voltage writing to each pixel in either field, and performing negative voltage writing to the pixel in the other field, the percentages of positive-voltage-holding pixels and negative-voltage-holding pixels in one row of pixels are each 50 percent.
A projector of the above type can De connected to various types of image sources such as a personal computer and a television receiver. Image signals (video signals) supplied from the image sources differ for each image source in terms of, for example, the number of horizontal lines. If a driving method of the related art is employed in a liquid crystal device, the driving method only needs to convert a supplied image signal into a form adapted for driving pixels of the liquid crystal device. However, when the above region-scanning driving is employed, the following problem occurs. Specifically, in a case such as switching of image sources, when a pixel is noted, a period in which the pixel holds a positive voltage and a period in which the pixel holds a negative voltage have a difference, so that a DC (direct current) component is applied to cause liquid crystal of the pixel to deteriorate.
Similarly to sticking occurring on a fluorescent screen of a CRT (cathode-ray tube), due to deterioration in liquid crystal, a fixed image unrelated to an image to be displayed may appear. Accordingly, similarly to the case of the CRT, this display phenomenon caused by deterioration in liquid crystal is also called “sticking”.